The present invention relates to an electrical voltage supply system, in particular, in a vehicle electrical system.
For generating the electrical energy required in a motor vehicle, usually claw-pole generators are used today. Such claw-pole generators are three-phase alternators whose output voltage is rectified with the assistance of a rectifier bridge. The rectified current is then used for supplying the electric loads of the vehicle as well as for charging the battery. The output voltage of the claw-pole generator is controlled with the assistance of a voltage regulator in such a way that it is approximately constant, independently of the alternator speed. The voltage level which is adjusted is usually selected in such a way that it is optimally suited for charging the battery.
In vehicle electrical systems having a plurality of loads, the problem exists that the voltage of 14V usually supplied by the alternator is not sufficient for the power supply. Since, usually, there are also loads in the vehicle electrical system which need a higher voltage than 12V and 14V respectively, it is known to use alternators which are regulated to a higher voltage than 12V and 14V respectively. Thus, it is suggested, for example, in European Patent Application No. 0 325 205 to regulate the alternator in a vehicle electrical system to 48V, and to derive the usual voltage of the electrical system of 12 and 14V respectively from this higher voltage with the assistance of voltage transformers. As the main battery in this known vehicle electrical system, a high-voltage battery is used which is charged with the assistance of the voltage of 48V supplied by the alternator. To the loads having a lower supply voltage, back-up capacitors can be assigned which stabilize the transformed voltage. The known design approach has the disadvantage that, instead of a conventional battery having a rated voltage of 12V, a battery suited for 48 volts must be used.
The electrical voltage supply system according to the present invention has the advantage that a conventional 12V-battery can be used as the main battery, that the alternator can be regulated to an output voltage of approximately 14V, and that a higher voltage is made available for the vehicle electrical system. In this manner, it is possible to build up a vehicle electrical system having a higher voltage.
It is particularly advantageous that energy can be drawn from the vehicle electrical system having a higher voltage even when the alternator is at rest. For this purpose, a dc-to-dc converter is provided which transforms the voltage of the battery into a higher voltage. The inductive component which is needed in such a dc-to-dc converter is in a particularly advantageous manner the alternator itself whose phase winding inductances are used as a choke (choke inductors) for the dc-to-dc converter which works as a step-up transformer.
Since it is possible to connect conventional loads of the vehicle electrical system to the battery in the usual manner, the electrical voltage supply system according to the present invention can be used to build up a vehicle electrical system which, in an advantageous manner, is composed of an electrical system having a conventional voltage and an electrical system having a higher voltage. For supplying the conventional loads and the battery, a further alternator can be advantageously used which is regulated to an output voltage of approximately 14V.
The interconnection of the alternator can be adapted to the requirements or conditions in an advantageous manner. The alternator is expediently operated in a star connection in order that the phase winding inductances are usable for the dc-to-dc converter. However, it is also possible to operate the alternator in a delta connection; in this case, additional circuit measures are to be taken. In any case, due to the advantageous arrangement, or rather interconnection of the alternator with the battery of the vehicle electrical system and the loads of the vehicle electrical system having a higher voltage, it is possible to ensure that the loads of the vehicle electrical system are supplied with voltage even when the alternator is at rest. The rectifier bridge of the alternator can, in an advantageous manner, either have a conventional design, in this case, a step-up transformer having to follow; however, the rectifier bridge can also include components of the step-up transformer itself, in this case, individual rectifier elements having to be replaced with field-effect transistor.
In the following, the present invention is described on the basis of a claw-pole generator. In principle, the electrical voltage supply system according to the present invention can be built up using any alternators, i.e., independently of the number of phase winding inductances, alternators being conceivable which have less than three or more than three phase winding inductances.